The present invention relates generally to chemical vapor deposition, and in particular to methods for chemical vapor deposition including preheating of the chemical vapor deposition precursors, systems to perform the methods, and apparatus produced by such methods.
Semiconductor integrated circuits (ICs) contain individual devices that are typically coupled together using metal line interconnects and various contacts. In many applications, the metal lines are formed on a different level than the devices, separated by an intermetal dielectric, such as silicon oxide or borophosphosilicate glass (BPSG). Commonly used metal lines include aluminum, tungsten and copper, as well as combinations of these materials with refractory metals and silicon. Interconnects used to electrically couple devices and metal lines are formed between the individual devices and the metal lines. A typical interconnect is composed of a contact hole (i.e. opening) formed in an intermetal dielectric layer over an active device region. The contact hole is often filled with a metal, such as aluminum or tungsten.
Interconnects often further contain a diffusion barrier layer sandwiched between the interconnect metal and the active device region at the bottom of the contact hole. Such layers prevent intermixing of the metal and the material from the active device region, such as silicon. Reducing intermixing generally extends the life of the device. Passive titanium nitride (TiN) layers are commonly used as diffusion barrier layers. An example may include the use of titanium nitride interposed between a silicide contact and a metal fill within a contact hole. Further uses of diffusion barrier layers may include a barrier layer interposed between a polysilicon layer and a metal layer in a gate stack of a field effect transistor.
Titanium nitride is a desirable barrier layer because it is an impermeable barrier for silicon, and because it presents a high barrier to the diffusion of other impurities. Titanium nitride has relatively high chemical and thermodynamic stability and a relatively low resistivity. Titanium nitride layers are also often used as adhesion layers, such as for tungsten films. While titanium nitride can be formed on the substrate by physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques, CVD is often the technique of choice.
CVD is a process in which a deposition surface is contacted with vapors of volatile chemical compounds, generally at elevated temperatures. The compounds, or CVD precursors, are reduced or dissociated at the deposition surface, resulting in an adherent coating of a preselected composition. In contrast to physical deposition, CVD does not require high vacuum systems and permits a wide variety of processing environments, including low pressure through atmospheric pressure, and is an accepted method for depositing homogeneous films over large areas and on non-planar surfaces.
CVD is often classified into various types in accordance with the heating method, gas pressure, and/or chemical reaction. For example, conventional CVD methods include cold-wall CVD, in which only a deposition substrate is heated; hot-wall CVD, in which an entire reaction chamber is heated; atmospheric CVD, in which reaction occurs at a pressure of about one atmosphere; low-pressure CVD (LPCVD) in which reaction occurs at pressures from about 10xe2x88x921 to 100 torr; and plasma-assisted CVD (PACVD) and photo-assisted CVD in which the energy from a plasma or a light source activates the precursor to allow depositions at reduced substrate temperatures. Other classifications are known in the art.
In a typical CVD process, the substrate on which deposition is to occur is placed in a reaction chamber, and is heated to a temperature sufficient to drive the desired reaction. The reactant gases containing the CVD precursors are introduced into the reaction chamber where the precursors are transported to, and subsequently adsorbed on, the deposition surface. Surface reactions deposit nonvolatile reaction products on the deposition surface. Volatile reaction products are then evacuated or exhausted from the reaction chamber. While it is generally true that the nonvolatile reaction products are deposited on the deposition surface, and that volatile reaction products are removed, the realities of industrial processing recognize that undesirable volatile reaction products, as well as nonvolatile reaction products from secondary or side reactions, may be incorporated into the deposited layer. Integrated circuit fabrication generally includes the deposition of a variety of material layers on a substrate, and CVD may used to deposit one or more of these layers.
As an example, one LPCVD process combines titanium tetrachloride (TiCl4) and ammonia (NH3) to deposit titanium nitride. However, LPCVD titanium nitride using these precursors has a tendency to incorporate a large amount of residual ammonium chloride in the film. This residual ammonium chloride detrimentally effects the resistivity and barrier properties of the titanium nitride layer. Once exposed to air, the residual ammonium chloride will cause the titanium nitride layer to absorb water and to form particles, both undesirable effects. It is known that residual ammonium chloride can be reduced by the use of ammonia post-flow, or annealing in an ammonia atmosphere, subsequent to deposition. However, such post-processing leads to reduced throughput and a higher risk of particle formation. It is also known that increased reaction temperatures can be used to reduce the incorporation of residual ammonium chloride. However, this, too, is detrimental as increased processing temperatures reduce the thermal budget available for subsequent processing and often lead to undesirable dopant diffusion.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods of chemical vapor deposition.
The various embodiments of the invention include chemical vapor deposition methods, chemical vapor deposition systems to perform the methods, and apparatus produced by such chemical vapor deposition methods. The methods involve preheating one or more of the reactant gases used to form a deposited layer. The reactant gases contain at least one chemical vapor deposition precursor. Heating one or more of the reactant gases prior to introduction to the reaction chamber may be used to improve physical characteristics of the resulting deposited layer, to improve the physical characteristics of the underlying substrate and/or to improve the thermal budget available for subsequent processing. One example includes the formation of a titanium nitride layer with reactant gases containing the precursors of titanium tetrachloride and ammonia. Preheating the reactant gases containing titanium tetrachloride and ammonia can reduce ammonium chloride impurity levels in the resulting titanium nitride layer, thereby reducing or eliminating the need for post-processing to remove the ammonium chloride impurity.
For one embodiment, the invention provides a method of depositing a layer of material on a substrate. The method includes heating a reactant gas containing at least one chemical vapor deposition precursor to a temperature within approximately 150xc2x0 C. of an auto-reaction temperature of each chemical vapor deposition precursor of the reactant gas, introducing the heated reactant gas to a reaction chamber containing the substrate, and reacting the reactant gas in the reaction chamber. Reacting the reactant gas involves reaction of the chemical vapor deposition precursors to deposit the layer of material on the substrate. It is recognized that additional compounds may be incorporated into the layer of material, such as nonvolatile reaction products from side reactions deposited in the layer of material as well as volatile reaction products from desired or side reaction products entrapped in the layer of material.
For another embodiment, the invention provides a method of depositing a layer of material on a substrate. The method includes heating a reactant gas containing at least one chemical vapor deposition precursor to a temperature below an auto-reaction temperature of each chemical vapor deposition precursor of the reactant gas, combining the heated reactant gas and at least one additional reactant gas, introducing the combined gases to a reaction chamber containing the substrate, and reacting the combined gases in the reaction chamber. Reacting the combined gases deposits at least the layer of material on the substrate. For yet another embodiment, the additional reactant gases are also heated prior to introduction to the reaction chamber.
For a further embodiment, the invention provides a method of depositing a layer of titanium nitride on a substrate. The method includes heating a first reactant gas containing titanium tetrachloride to a first temperature and heating a second reactant gas containing ammonia to a second temperature. The first and second temperatures are each below an auto-reaction temperature of titanium tetrachloride and ammonia. The method further includes combining the heated first and second reactant gases, introducing the combined first and second reactant gases to a reaction chamber containing the substrate, reacting the first and second reactant gases in the reaction chamber to produce titanium nitride, and depositing the titanium nitride on the substrate.
For another embodiment, the invention provides a chemical vapor deposition system. The chemical vapor deposition system includes a gas source, a reaction chamber, a gas conduit coupled between the gas source and the reaction chamber, a heater, a gas flow temperature sensor coupled to the gas conduit between the heater and the reaction chamber, and a control system coupled to the gas flow temperature sensor and the heater. The control system is adapted to adjust energy input from the heater to the gas conduit in response to data from the gas flow temperature sensor. For yet another embodiment, the chemical vapor deposition system further includes a gas flow control valve coupled to the gas conduit. For this embodiment, the control system is further coupled to the gas flow control valve and is further adapted to control an opening of the gas flow control valve in response to data from the gas flow temperature sensor.
Further embodiments of the invention include deposition methods and chemical vapor deposition systems of varying scope, as well as apparatus making use of such deposition methods and chemical vapor deposition systems.